link.strength()

link.strength([strength])

If strength is specified, sets the strength accessor to the specified number or function, re-evaluates the strength accessor for each link, and returns this force. If strength is not specified, returns the current strength accessor, which defaults to:

function strength(link) {
  return 1 / Math.min(count(link.source), count(link.target));
}

Where count(node) is a function that returns the number of links with the given node as a source or target. This default was chosen because it automatically reduces the strength of links connected to heavily-connected nodes, improving stability.

The strength accessor is invoked for each link, being passed the link and its zero-based index. The resulting number is then stored internally, such that the strength of each link is only recomputed when the force is initialized or when this method is called with a new strength, and not on every application of the force.